As disclosed in the Japanese Laid-Open Patent Application No. 311667/2003, conventionally electric motor such as servomotor has been employed as an actuator for moving a joint of a robot, since motors are easily available. However, there resides such a disadvantage in motors that the entire size of robot tends to become larger. Since motors weigh considerably, design of mechanical strength of robot is also important. The fluid cylinders such as air cylinder have such advantages that, compared to motors, smaller in weight, simple in structure and easy to maintain. The fluid cylinder is estimated as useful as an actuator for robots.
Patent document 1: Japanese Patent Application Laid-Open 311667/2003